Hi,
I am working with CAMB1.0.12, and I am trying to modify some physics. My first aim is to artificially suppress the growth of only the small scale (large wavenumber k) density perturbations of the cold dark matter species.
I tried to modify the “equation of motion” for the CDM in the “equations.f90” file, which normally reads:
Clxdot=k*z
I then recompiled using “python setup.py make”, but when I run the simulation, I see no difference in the result, even when replacing the above equation with Clxdot=k*z*0.
My questions are:
1) I should see some effect of a modified equation of motion, so what am I doing wrong? Am I modifying the wrong equation, or in the wrong file? Is there a problem with recompiling?
2) In “equations.f90” I found the baryon equation of motion: clxbdot=k*(z+vb). But this equation seems different than what I expected from e.g. Eq. 530 in arXiv:astroph/9804301v2. Also “dz” seems different than expected from Eq. 531. I have looked around but didn’t find exactly the underlying theory behind “equations.f90”. Am I missing something here?
3) Is there a simple intuitive argument why the evolution of perturbations of one species (e.g. Delta_baryon) is independent of the choice of Omega_bh^2 or Omega_cdm^2? From Eq. 622 in in arXiv:astroph/9804301v2 I would expect that the evolution of each species depends on the SUM of all densities and pressures of all other species.
All feedback is much appreciated,
Filip
CAMB: modifying equations.f90

 Posts: 2
 Joined: December 23 2019
 Affiliation: Ghent university

 Posts: 1962
 Joined: September 23 2004
 Affiliation: University of Sussex
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Re: CAMB: modifying equations.f90
Sounds OK, make sure you restart Python. The equations neglect some very small pressure terms (matter pressure is only important when coupled via derivatives so the last term in 530 is very small).
The evolution equations are not independent of those quantities (except in some limits).
The evolution equations are not independent of those quantities (except in some limits).

 Posts: 2
 Joined: December 23 2019
 Affiliation: Ghent university
Re: CAMB: modifying equations.f90
Hi,
I have now managed to change the equations of motion in the file "equations.f90", to compile and run  so far so good...
But I still have trouble locating the following equations found in ScalEqs.txt in the file "equations.f90":
drhob:=3*H(t)*(rhob(t)+pb(t))
drhoc:=3*H(t)*rhoc(t);
Can somebody point out exactly in which line of equations.f90 these calculations (namely the effect of expansion on the densities of each species) are carried out?
Thanks a lot,
Filip
I have now managed to change the equations of motion in the file "equations.f90", to compile and run  so far so good...
But I still have trouble locating the following equations found in ScalEqs.txt in the file "equations.f90":
drhob:=3*H(t)*(rhob(t)+pb(t))
drhoc:=3*H(t)*rhoc(t);
Can somebody point out exactly in which line of equations.f90 these calculations (namely the effect of expansion on the densities of each species) are carried out?
Thanks a lot,
Filip

 Posts: 1
 Joined: November 03 2023
 Affiliation: Universidad de Chile
Re: CAMB: modifying equations.f90
How did you do to compile before running? Is it enough to do "make" inside the fortran folder?

 Posts: 1962
 Joined: September 23 2004
 Affiliation: University of Sussex
 Contact:
Re: CAMB: modifying equations.f90
"python setup.py make" in the main folder will update an existing "pip install e" installation.